The invention relates to an apparatus for annealing a multilayer body, which has a first layer and at least one second layer, through uptake of a quantity of energy by the multilayer body involving uptake of a first partial quantity of the quantity of energy by the first layer and uptake of a second partial quantity of the quantity of energy by the second layer, having at least one energy source. An apparatus of this type is known, for example, from EP 0 662 247 B1. As well as the apparatus, the invention also proposes a process for annealing a multilayer body and a multilayer body of this type.
A multilayer body is produced, for example, by applying a functional layer to a substrate layer. To ensure that the functional layer and/or the substrate layer has a desired physical (electrical, mechanical, etc.) and/or chemical property, under certain circumstances it is necessary for the multilayer body or the layer and/or the substrate layer to be processed. The processing comprises, for example, annealing of the multilayer body in the presence of a gas (process gas).
A multilayer body is, for example, a large-area thin-film solar cell, in which an electrode layer comprising molybdenum and a functional copper-indium-diselenide (CIS) semiconductor layer are applied to a substrate layer of glass. This thin-film solar cell is produced in a two-stage process according to EP 0 662 247 B1. In a first stage, the following elements are applied in layer form, in order, to the glass substrate layer:molybdenum, copper, indium and selenium. In a second stage, the multilayer body obtained in this way is annealed, leading to the formation of the copper-indium diselenide semiconductor layer.
For annealing, the multilayer-body is arranged in a closed container made from graphite. During the annealing, a defined partial pressure of gaseous selenium is formed in the interior of the container, the layers which have been applied to the glass being brought into contact with the gaseous selenium.
During the annealing, the multilayer body takes up a quantity of energy, each layer being supplied with a partial quantity of the quantity of energy. The annealing takes place, for example, at a heat-up rate of 10° C./s. The energy source for the quantity of energy which is used is a halogen lamp. The halogen lamp is used to irradiate the graphite container and thus to heat the container. An operation of this type is particularly efficient, since graphite which acts, as it were, as a “black body radiator”, has a high absorption capacity for electromagnetic radiation, in particular for radiation in the spectral region of the halogen lamp. The quantity of energy absorbed by the graphite is fed to the multilayer body by heat radiation and/or heat conduction. The container therefore functions as a secondary energy source or as an energy transmitter.
Graphite has a high emission capacity and a high thermal conductivity. When the multilayer body is resting on a base of the container, the quantity of energy is supplied to an underside of the multilayer body substantially by heat conduction. A quantity of energy is fed to an upper side of the multilayer body by heat radiation.
On account of an asymmetric layer structure of the multilayer body and/or a different quantity of energy being supplied to the top side and the underside of the multilayer body, a high heating rate may lead to inhomogeneous, i.e. non-uniform annealing of the layers of the multilayer body. Temperature inhomogeneity may form in the thickness direction of the multilayer body and, given a coefficient of thermal expansion of a material of a layer which is not zero, may lead to mechanical stress within the layer and/or the multilayer body. This mechanical stress may cause the layer and/or the multilayer body to crack or fracture. The mechanical stress may also lead to deformation (distortion) of the multilayer body. In the case of a substrate layer made from glass, the deformation is generally transient, i.e. disappears again after the annealing. The deformation may also be permanent. In this case, the deformation does not disappear again. This is the case if a softening point of the substrate layer (e.g. of glass) is exceeded during the annealing and an (internal) mechanical stress and/or an external force becomes active.
The larger the area of the multilayer body and the higher the annealing rate (heating rate, cooling rate), the more difficult it becomes to deliberately influence temperature inhomogeneities in the multilayer body during the annealing of the multilayer body and the greater the likelihood of an undesirable mechanical stress occurring.
It is an object of the invention to demonstrate how temperature homogeneity or temperature inhomogeneity can be deliberately influenced during the annealing of a large-area multilayer body with a high annealing rate.